Digital cellular wireless communication systems employing the DS/CDMA (Direct Sequence Code Division Multiple Access) method are being developed as next-generation mobile communication systems enabling wireless multimedia communication. An array antenna for wireless base stations is effective for increasing the subscriber capacity of a mobile communication system adopting DS/CDMA and for reducing the transmission power of mobile stations.
This invention relates to a CDMA receiver comprising an array antenna, and a searcher for the above CDMA receiver. In particular, this invention relates to a searcher which synthesizes a phase-adjusted voltage profile, obtained by performing correlation computation of signals received by each antenna element of the array antenna, and which detects the path timing for multiple paths using this synthesized voltage profile, and also relates to the CDMA receiver of a wireless base station using this searcher.
In a mobile communication system using DS/CDMA, (1) the RAKE reception method, (2) the array antenna method, and (3) transmission power control, are well-known as component technologies to increase subscriber capacity and reduce the transmitting power of mobile stations. The RAKE reception method attempts to improve characteristics by utilizing signals arriving via each path among multiple paths. Mobile communication is characterized in that radio waves output from a transmitter follow a number of propagation paths (multiple paths, or multipath), with different path lengths, to arrive at a receiver. In the RAKE reception method, signals propagating over different paths are identified, reliability weightings are assigned, and synthesis performed to improve the signal-to-noise ratio (SNR). In the array antenna method, gain is improved by narrowing the directionality pattern, and the SNR is improved by reducing interfering signals. In transmission power control, the transmitting power of the transmitter is controlled such that the received signal SNR is held constant.
FIG. 6 is a diagram of the configuration of the receiver of a DS/CDMA wireless base station employing an array antenna and RAKE reception method. In the FIG. 1 is an array antenna for reception, having N antenna elements 11 to 1N (in the figure, four). 21 to 2N are reception circuits (RV) which amplify the RF signal which is the antenna output, and then perform frequency conversion to a baseband signal (RF→IF conversion), execute quadrature detection (QPSK demodulation) for the baseband signal, and performs A/D conversion to output the demodulated signal in digital form. Through quadrature detection, an in-phase component I and a quadrature component Q are obtained. 3 is a searcher, 41 to 4M are finger portions provided corresponding to each of the multiple paths, 5 is a RAKE synthesis portion which synthesizes the output of each of the finger portions, and 6 is a discrimination portion which discriminates between “1” and “0” among the received data, based on output from the RAKE synthesis portion. The array antenna 1 and reception circuit 2 are provided in common for all channels, and the searcher 3, finger portions 4, RAKE synthesis portion 5, and discrimination portion 6 are provided for each channel.
FIG. 7 explains the frame format for uplink signals from a mobile station to the base station. One frame is 10 msec, and comprises 15 slots, S0 to S14. The data portion is mapped onto the I channel in QPSK modulation, and the part other than the data portion is mapped onto the Q channel in QPSK modulation. Each of the slots in the I channel over which the data portion is transmitted comprises n bits, where n varies according to the symbol rate. Each of the slots of the Q channel transmitting control data comprises 10 bits (10 symbols), and the symbol rate is constant at 15 ksps; the pilot PILOT, transmission power control data TPC, transport format combination indicator TFCI, and feedback information FBI are transmitted. PILOT is used in synchronous detection on the receiving side and in SIR measurements; TPC is used in transmitting power control; TFCI transmits the data symbol rate and the number of bits per frame; and FBI controls the transmission diversity at the base station. This I channel data and Q channel data is subjected to QPSK modulation and is transmitted on the transmitting side, and on the receiving side is QPSK demodulated and restored.
As shown in FIG. 8, in mobile communications the reception level in the receiver of signals sent from the transmitter varies depending on the multipath, and the time of arrival at the receiver also differs. The searcher 3 measures the profile of power received by the antenna (change with time in the power level, called the delay profile), and referring to this delay profile, detects multiple paths among the multipath signals MP1 to MPM higher than a threshold value, identifies the times of occurrence T1 to TM of each of these multiple paths or the delay times from a reference time, and inputs, to the finger portions 41 to 4M corresponding to each path, timing information for the start of despreading and delay time information.
In the searcher 3, the matched filter 3a uses the spreading code for the channel to extract and output the pilot signal component (one symbol's worth) for the channel from the signal received by the prescribed antenna element 1N. That is, when a direct sequence signal which has been affected by multipath is input, the matched filter 3a outputs a pulse train having a plurality of peaks corresponding to arrival times and signal strengths (FIG. 8). The pilot signal is affected by noise on the communication path, and so the averaging circuit 3b adds the voltages of the correlated output for each symbol over a prescribed pilot signal segment (one-slot segment) in order to reduce the effect of this noise, in an attempt to improve the SNR of the received signal in path timing detection. The power calculation portion, not shown, converts the output signal of the averaging circuit 3b into power, and the delay profile RAM 3c stores the delay profile converted into this power and the path detection portion 3d detects each path among the multiple paths and the delay times T1 to TM, referring to the delay profile stored in RAM, and inputs the timing information for the start of despreading and delay time information to the finger portions 41 to 4M corresponding to each path.
The finger portions 41 to 4M corresponding to each of the multiple paths are of the same configuration, and have delay time adjustment portions 4a1 to 4aN, despreading circuits 4b1 to 4bN, a beam former 4c, weighting coefficient computation portion 4d, synchronous detection circuit 4e, and error computation portion 4f. 
The delay time adjustment portions 4a1 to 4aN adjust the delay time of signals received from each antenna element corresponding to a path (in actuality, I channel signals and Q channel signals) based on delay time information, to coordinate the timing of signals for different paths. The despreading circuits 4b1 to 4bN multiply the spreading code assigned to the channel by the output signals of the delay time adjustment portions 4a1 to 4aN, based on despreading start timing information, to perform despreading. The beam former 4c having N multipliers 4c1 . . . 4cN (in FIG. 6) and an adder 4ca forms antenna directionality by adding a weighting to the output signals of each of the despreading circuits. That is, if the despreading circuit output for the kth symbol of the nth antenna element is υn(k) and the weighting coefficient is ωn(k), then the beam former 4c outputs the weighting synthesis signal y(k) expressed by the equation
                              y          ⁡                      (            k            )                          =                              ∑                          n              =              1                        N                    ⁢                                                    ω                n                            ⁡                              (                k                )                                      ⁢                                          υ                n                            ⁡                              (                k                )                                                                        (        1        )            
The weighting coefficient computation portion 4d computes the weightings w1(k) to wN(k) in the beam former 4c from the LMS (Least Mean Square) adaptive algorithm, which is well known. Channel estimation is performed by the synchronous detection circuit 4e based on phase differences between the pilot signal contained in the received signal and the known pilot signal, and the complex conjugate ξ* of the channel estimate ξ is multiplied by the weighting synthesis signal (bean farmer output) to perform phase rotation processing (synchronous detection). The RAKE synthesis portion 5 synthesizes the synchronous detection output of each of the finger portions, and the data discrimination portion 6 performs data discrimination based on the RAKE synthesis output. This data discrimination result is fed back in order to determine weighting coefficients using the LMS adaptive algorithm.
The error computation portion 4f performs operations on the weighting synthesis signal y(k) and its error e(k) with the data discrimination results (pilot signal d(k)), and inputs the result to the weighting coefficient computation portion 4d. In the data discrimination result, the phase rotates in the synchronous detection circuit 4e by the amount of multiplication by the complex conjugate of the channel estimate ξ. In the error computation portion 4f, then, the multiplier 4f1 multiplies the discrimination result by the channel estimate ξ to return the phase to its original value, and in the error computation portion 4f2 the error e(k) between the discrimination result with phase returned (pilot signal) and the weighting synthesis signal is computed, and the result input to the weighting coefficient computation portion 4d. 
The weighting coefficient computation portion 4d computes the weightings w1(k) to wN(k) from the LMS adaptive algorithm. That is, if the weighting coefficient for the (k+1)th symbol of the nth antenna element is wn(k+1), and the weight coefficient of the kth symbol is wn(k), then wn(k+1) can be computed from:wn(k+1)=wn(k)+μ e★(k)υn(k)  (2)e(k)=ξ·d(k)−y(k)  (3)
Here μ is the step coefficient, e(k) is the error signal given by eq. (3), ★ indicates a complex conjugate, ξ is the channel estimate, and d(k) is the pilot signal used as the reference signal.
As described above, by adding a weighting to the signals received from each antenna element using the beam former, antenna directionality can be formed, so that gain can be increased and interference signals can be reduced. And by adopting the RAKE reception method, each of the signals arriving via multiple paths can be utilized, so that the SNR can be improved.
FIG. 9 explains uplink circuit closed-loop transmission power control. In the mobile station 11, the spreading modulation portion 11a uses a spreading code according to the prescribed channel specified by the base station to perform spreading modulation of transmission data, and the power amplifier 11b performs quadrature modulation after spreading modulation, frequency conversion and other processing, amplifies the input signal, and transmits the signal to the base station 12 from the antenna. At the base station 12, the despreading portions 12a of the finger portion corresponding to each path performs despreading processing for delayed signals arriving via the allocated path, and the RAKE demodulation portion 12b synthesizes the signals output from each finger, and discriminates between “1” and “0” among the received data based on the synthesized signals.
The SIR measurement portion 12c measures the power ratio (SIR, Signal Interference Ratio) of the received signal to the interference signal, including thermal noise. The comparison portion 12d compares the target SIR and the measured SIR, and if the measured SIR is larger than the target SIR, creates a command to lower the transmission power by 1 dB using a TPC (Transmission Power Control) bit, whereas if the measured SIR is small, creates a command to raise by 1 dB the transmission power using the TPC bit. The target SIR is the SIR value necessary to obtain a BER of, for example, 10−3 (corresponding to the occurrence of an error once every thousand times), and is input to the comparison portion 12d from the target SIR setting portion 12e. The spreading modulation portion 12f performs spreading modulation of the transmission data and TPC bit. After spreading modulation, the base station 12 performs D/A conversion, quadrature modulation, frequency conversion, power amplification and other processing, and transmits to the mobile station 11 from the antenna. The despreading portion 11c of the mobile station 11 performs despreading processing of the signal received from the base station 12, the RAKE demodulation portion lid demodulates the received data and TPC bit, and the transmission power of the power amplifier 11b is controlled according to the command indicated by the TPC bit.
In the receiver of a DS/CDMA wireless base station employing an array antenna, it is possible to raise the gain and reduce interference signals; but if transmission power is controlled, the reception SNR per element of the antenna 1 drops compared with a receiver not employing an array antenna. If the reception SNR per element of the antenna 1 falls, the effect of noise is increased in the searcher 3, and there is the problem that path searches cannot be performed accurately. In order to maximize the effect of the RAKE reception method, it is essential that path searches be performed accurately; the above problem complicates the introduction of an array antenna to a DS/CDMA mobile communication system.